Nightmares are one of the more loathsome aspects of the human condition. But there is a science behind these terrifying visions. And there are actually a few simple ways to keep the bad dreams away.

What are nightmares?

Nightmares are, of course, a kind of dream — a very nasty kind of dream.

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Scientists who study dreams, what are called oneirologists, aren't entirely sure where dreams originate in the brain or if a single point can be isolated. What they've discovered, however, is that 75% of all dreams elicit negative emotions, or contain some kind of negative content (which is disappointingly high if you ask me). And every once in awhile that content gets a bit too dark and disturbing, jolting us awake with a pounding heart.

Indeed, nightmares are often described as a series of frightening images, ideas, emotions, and sensations that spontaneously (and uncontrollably) arise during sleep. People can experience any number of emotions during a bad dream, including sadness, depression, anger, guilt — and especially feelings of fear and anxiety.

Nightmares also tend to be very realistic, something that gives them that added bit of spice. They often feature disturbing imagery or themes that are so awful and terrifying that they force us awake. These feelings tend to linger, often making it hard to fall back asleep.

The content of nightmares varies widely from person to person, but there are some common themes. Perhaps the most archetypal nightmare is the one in which we're not able to run fast enough while we're being chased. Other common nightmares include falling, or revisiting a traumatic event. Children tend to have dreams in which they are chased by an animal or a fantasy figure.

A nightmare, like any other kind of dream, can last for a few minutes, or drag on for upwards of 20 minutes. And because REM periods get longer as the night progresses, most nightmares happen in the early morning.

Clinically speaking, virtually everyone experiences a nightmare from time to time, and they are considered completely normal. They are most common in children, and typically peak in frequency from age three to eight. Around 5 to 10% of adults have nightmares at least once a month or more.

It's worth noting that nightmares are different from night terrors, which tend to happen early in the night are are primarily experienced as feelings.

What causes them?

Most oneirologists theorize that dreams are epiphenomenon of consciousness and sleep, and that they don't serve any kind of purpose.

But some evolutionary psychologists, including Antti Revonsuo, believe that dreams — and especially nightmares — may actually serve an evolutionary purpose. His contention is that nightmares are a kind of "threat simulation" that prepares people for the perils of the real world — or at least the threats that were faced by our ancestors.

By priming dreamers with these negative experiences, Revonsuo says we're put into a kind of rehearsal for real life. People who dream, he argues, have that extra bit of experience — even if it is fictional. And interestingly, he argues that nightmares are as bad as they are in order to help us cope with more realistic adversity.

Evolutionary psychology aside, the most proximate cause of nightmares is not known. Neuroscientists still need to develop a more sophisticated model of consciousness and the dream-state to make this sort of determination. What they do know, however, is that high frequency nightmares tend to run in the family, which could indicate a possible genetic link.

Now, while we don't necessarily know the exact mechanics of why bad dreams happen, we do know that they can be triggered by emotional and physiological factors.

People who are ill or in a fever tend to experience nightmares more than usual, as do individuals who are experiencing withdrawal symptoms from drugs. Some antidepressants and blood pressure medications have been known to cause them as well.

And it seems the old adage is true: Going to bed soon after eating will heighten your chance of having a nightmare owing to an increase in metabolism — a signal to your brain to be more active.

Bad dreams can also be triggered by sleep disorders such as sleep apnea and restless leg syndrome.

There are also the stresses of daily life to consider. Nightmares tend to happen during transitional or tumultuous periods in our lives — like changing a job, moving, a pregnancy, or financial concerns. They can also be triggered by more serious events, such as the loss of a loved one, a serious accident, or witnessing a traumatic event.

For most people, nightmares don't happen frequently enough to pose a problem. We get them, and move on. But for some people, they happen often enough to pose a definite health risk — a condition that can result in depression and increased anxiety. It is recommended that people who suffer from chronic nightmares go see their doctor as there are a number of treatments available.

One technique that's increasingly being used is "imagery rehearsal treatment" where individuals are encouraged to alter the endings of their nightmares while they're awake. It's a form of cognitive therapy in which people can create an alternative, less distressing outcome to their dreams. Follow-up studies have shown that these kinds of therapies are effective, with upwards of 70% of people claiming to have experienced benefits (including people with PTSD and insomnia).

Similarly, chronic bad dreamers are told to write down the details of their nightmare, or to draw or paint them. They're also encouraged to talk in fantasy to the characters of their dreams. And in all cases, they are told to imagine a more pleasant ending.

Failing that, there are also pharmaceuticals that can help. The most effective drug is called prazosin, which is used extensively to help patients with PTSD. The drug is also typically used to treat high blood pressure, anxiety, and panic disorders.

And lastly, there are some simple (and commonsense) things you can do to stave of nightmares, including relaxation techniques (like yoga, meditation, and breathing exercises), physical exercise, and ensuring that your bedroom is a relaxed and stress-free environment. It's also recommended that no food, alcohol, caffeine, or nicotine be taken before bed.